1. Field of the Invention
The present invention relates to a method for hydrogenating a solvent-refined coal obtained by treating coal with a solvent, thereby to convert the solvent-refined coal into a liquid substance. More particularly, the present invention relates to a method for converting a solvent-refined coal fraction containing no substantial amount of preasphaltene into a liquid hydrocarbon oil by heating it under hydrogen pressure in a fixed bed.
2. Description of the Prior Art
In view of the recent unfavorable situation surrounding energy and resources, it is desired to develop a liquid fuel which can be substituted for petroleum. In view of abundant coal reserves, it is particularly important to establish a technique for liquefying coal.
In general, petroleum is a liquid composed mainly of aliphatic hydrocarbon compounds, whereas coal has a small ratio of hydrogen to carbon, as compared with petroleum, and it is a solid substance composed mainly of condensed aromatic compounds. However, when coal is treated with high pressure hydrogen together with a hydrocarbon solvent, preferably a solvent having a high hydrogen donative property, at a high temperature in the presence or absence of a catalyst, a part of the condensed aromatic compounds will be hydrogenated, whereby the coal becomes soluble in a solvent.
Such a method for liquefying coal is usually called a solvent-refining method. If the reaction solution obtained by the treatment is distilled, liquefied light oil is obtainable as a distillate together with the solvent thereby recovered. Whereas, as the distillation residue, carbonaceous material which is solid at room temperature and which is called a solvent-refined coal, is obtained. Such a solvent-refined coal has a boiling point of at least 400.degree. C. under atmospheric pressure, and it melts when heated to a temperature of about 150.degree. C.
The solvent-refined coal thus obtained is soluble in a solvent such as quinoline in its major portion. However, it still contains insoluble components such as a carbonaceous substance insoluble in the solvent and an ash content present in the starting material coal, or a catalyst in the case where the catalyst is used in a suspension bed system. These insoluble components may usually be removed from the solvent-refined coal by a method such as sedimentation separation. The solvent-refined coal may be used by itself as a fuel, but it may also be used as a carbon material such as a substitute for caking coal for coke.
In order to obtain a liquefied oil in high yield in the above-mentioned solvent-refining liquefaction method wherein coal is subjected together with a solvent to hydrogenation treatment to obtain the solvent-refined coal and the liquefied oil, it is conceivable to facilitate the hydrogenolysis of the solvent-refined coal into the liquefied oil by employing severe reaction conditions. However, by such a method, it is likely that the resulting liquefied oil undergoes further decomposition and gasification, whereby the yield of the liquefied oil rather tends to decrease.
As a method to avoid this problem, there is a method wherein the resulting solvent-refined coal is separated from the liquefied oil, and then the solvent-refined coal is again subjected to hydrogenation treatment to convert it into a liquefied oil.
It is well known that a catalyst composed of a combination of cobalt, nickel, molybdenum, tungsten, etc., or red mud, iron ore, etc. comprising iron as the main component, is effective in the hydrogenation treatment of such a solvent-refined coal.
However, the solvent-refined coal as the feed material contains highly unsaturated polycondensation products or ash contents which deactivate the catalyst. Accordingly, if the solvent-refined coal itself is used for the hydrogenation treatment, it is difficult to maintain the catalytic activity for a long period of time even when a catalyst composed of expensive components such as cobalt, nickel, molybdenum or tungsten is used. On the other hand, if a catalyst comprising inexpensive and abundantly available iron as the major component, is used instead of the catalyst composed of the above-mentioned expensive components, the catalytic activity will be considerably inferior as compared with the catalyst composed of cobalt, nickel, molybdenum or tungsten, whereby the yield of the liquefied oil will be low.
Under the circumstances, the present inventors have made extensive study to find out the causes for the deactivation of the catalyst and to find out a method for prolonging the catalytic activity of the highly active catalyst composed of cobalt, nickel, molybdenum or tungsten, and have found that the solvent-refined coal obtained by a conventional deasphalting operation still contains a substantial amount of preasphaltene components, and such preasphaltene components are the major factor for the deactivation of the catalyst, and that by using a solvent-refined coal fraction containing no substantial amount of preasphaltene components, it is possible to maintain the catalytic activity for an extended period of time even when the catalyst composed of such expensive components is used, whereby the yield of the liquefied oil can be increased over the case where the solvent-refined coal is employed by itself. The present invention has been accomplished based on these discoveries.